1. Field of the Invention
The present invention relates to a ceramic turbo charger rotor comprising a blade portion including a shroud tip portion, a back plate portion arranged rear of the blade portion and a shaft portion extending from the back plate portion at an opposite side with respect to the wing portion, and relates to a method of manufacturing the same.
2. Related Art Statement
As for automobile parts, use is made of ceramic materials having characteristics such as an excellent high temperature strength, an excellent thermal resistance and a light weight as compared with metal materials. Especially, it is well known that ceramic turbo charger rotors made of silicon nitride show superior characteristics of high temperature strength, thermal resistance and reliability.
Generally, since turbo charger rotors made of ceramic materials have complicated shapes, a rotor having the blade portion including the shroud tip portion, the back plate portion and the shaft portion is manufactured in the following manner. First, the rotor is formed by using an injection molding method and the formed body is preliminarily heated to eliminate organic binders and the like. Then, the thus preliminarily heated body is sintered under a condition such that the shaft portion thereof is inserted into a cylindrical holder for a support, and the sintered body is worked into a final shape.
In this manufacturing method, it is necessary to grind not only the shaft portion which is to be inserted into a metal member and the shroud tip portion of the blade portion but also a rear portion of the blade portion and the back plate portion, as the reasons described below.
(1) In the metal turbo charger rotor, it is considered that a position of the rear portion of the blade portion must be controlled strictly so as to obtain a good acceleration responsibility. This consideration is maintained in the ceramic turbo charger rotor.
(2) Since the back plate portion is sometimes broken during a rotation examination if the rear portion and the back plate portion are in an as fired state, it is necessary to grind the rear portion and the back plate portion so as to make them stronger.
(3) Since a standard surface for use in working the rear portion and installation of the metal member must be arranged on the back plate portion, it is necessary to grind the rear portion of the blade portion.
However, since ceramic materials have harder and more brittle characteristics than those of metal materials, especially since the back plate portion has a complicated shape such as an oval shape or a tapered shape to reduce generation of stresses, the working of the ceramic turbo charger rotor such as grinding and polishing is very difficult and becomes expensive. As a result, a total cost of manufacturing the ceramic turbo charger rotor becomes expensive as compared with the metal turbo charger rotor.
In this regard, in order to reduce transformation of the ceramic sintered body and a decrease in strength, there are disclosed a method of effecting an isostatic pressing for the formed body before the sintering in Japanese Patent Publication No. 62-27034 and a method of sintering for reducing vaporization and decomposition of the binders in Japanese Patent Publication No. 61-3304. However, both of these references do not disclose a sintered body which needs no working.
Moreover, when the rear portion of the blade portion is worked, chipping is liable to be generated at a boundary portion between the shroud tip portion and the rear portion, and thus it is necessary to work and smooth the boundary portion to obtain a dull boundary portion. Further, if the ceramic turbo charger rotor, the rear portion of which is not worked, is rotated to effect a proof examination, the ceramic turbo charger rotor is often broken.